A significant cost in the hot working of iron and steel is yield loss due to the continuous oxidation of surfaces in the forming or rolling processes. At many stages in these operations, iron oxide (also called scale), is knocked off intentionally either through mechanical or high pressure water means to prevent “rolled in scale” or “imprinted scale” defects. Yield loss of the finished product occurs not only in the removal of scale, but under storage conditions. In many cases, plate, coils, tubular goods, long products and shapes are stored in unprotected environments. Low temperature iron oxides (rust) are formed when these products are left out in the environment, resulting in additional yield loss.
One method by which to reduce scale and rust on a substrate such as a ferriferous substrate is to form a conversion coating, such as an iron phosphate coating, on the surface of the substrate. Cold temperature conversion coatings are often formed at 60° C. by reacting a 5% solution of a conversion coating composition to form 25 mg/ft2 (after 30 seconds in a bath) of iron phosphate which protects the substrate from iron oxide and provides a lubricating surface for downstream operations and/or helps paint to adhere to the surface. Such cold temperature coatings generally must be formed using a submersion tank or a spray system involving a long run of spray zones to build up an effective amount of coating.
There is a need for methods and compositions for efficiently forming conversion coatings on ferriferous surfaces at high temperatures and high conversion rates without the need for submersion.